Synthetic gene for D4 dopamine receptors

ABSTRACT

A modified gene coding for the human D4 dopamine receptor has been synthesized by chemo-enzymatic methods. The nucleotide sequence of the D4 dopamine receptor gene was changed to reduce the G+C content and to eliminate intronic sequences, while maintaining the published amino acid sequence. Using gene splicing by overlap extension and PCR amplification of long oligonucleotides (&gt;200 bases), 3 synthetic fragments of about 400 base pairs each were amplified, from which the full length gene was assembled. Stable expression of this gene has been achieved in CHO-K1 cells, using an inducible expression system, and in HEK293 cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 08/086,439, filed 1 Jul. 1993, now U.S. Pat. No. 5,468,615.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to dopamine receptors and the genes corresponding to such receptors. In particular, it relates to a synthetic dopamine receptor having drug dissociation properties similar to those properties characteristic of the human dopamine receptor D4.

2. Background

Dopamine is a neurotransmitter that participates, in a variety of different functions mediated by the nervous system, including vision, movement, and behavior. See generally Cooper, J. et al. The Biochemical Basis of Neuropharmacology, 161-195 (Oxford University Press, NY 3d Ed. 1978). The diverse physiological actions of dopamine are in turn mediated by its interaction with two of the basic types of G protein-coupled receptors: D1 and D2, which respectively stimulate and inhibit the enzyme adenylyl cyclase. Kebabian, J. and Calne, D., Nature 277: 93-96 (1979). Alterations in the number or activity of these receptors may be a contributory factor in disease states such as Parkinson's disease (a movement disorder) and schizophrenia (a behavioral disorder).

A great deal of information has accumulated on the biochemistry of the D1 and D2 dopamine receptors, and methods have been developed to solubilize and purify these receptor proteins. See Senogles, S. et al., Biochemistry 25: 749-753 (1986); Gingrich, J. et al., Biochemistry 27: 3907-3912 (1988). The D1 dopamine receptor in several tissues appears to be a glycosylated membrane protein of about 72 kDa. Amlaiky, N. et al., Mol. Pharmacol. 31: 129-134 (1987); Niznik, H. et al., Biochemistry 27: 7594-7599 (1988). The D2 receptor has been suggested to have a higher molecular weight of about 90-150 kDa. Amlaiky, N. and Caron, M., J. Biol. Chem. 260: 1983-1986 (1985); Amlaiky, N. and Caron, M., J. Neurochem. 47: 196-204 (1986); Jarvie, J. et al., Mol. Pharmacol. 34: 91-97 (1988). Much less is known about a recently discovered additional dopamine receptor, termed D3. Sokoloff, P. et al. Nature 347: 146-151 (1990). Dopamine receptors are primary targets in the clinical treatment of psychomotor disorders such as Parkinson's disease and affective disorders such as schizophrenia. Seeman, P. et al, Neuropsychopharm. 1: 5-15 (1987); Seeman, P. Synapse 1: 152-333 (1987). The three different dopamine receptors (D1, D2, D3) have been cloned as a result of nucleotide sequence homology which exists between these receptor genes. Bunzow, J. R. et al. Nature 336: 783-787 (1988); Grandy, D. K. et al. Proc. Natl. Acad. Sci. U.S.A. 86: 9762-9766 (1989); Dal Toso, R. et al. EMBO J. 8: 4025-4034 (1989); Zhou, Q-Y. et al. Nature 346: 76-80 (1990); Sunahara, R. K. et al. Nature 346: 80-83 (1990); Sokoloff, P. et al. Nature 347, 146-151 (1990).

The antipsychotic clozapine is useful for socially withdrawn and treatment-resistant schizophrenics Kane, J. et al. Nature 347: 146-151 (1990)!, but unlike other antipsychotic drags, clozapine does not cause tardive dyskinesia Casey, D. E. Psychopharmacology 99: 547-553 (1989)!. Clozapine, however, has dissociation constants at D2 and D3 which are 3 to 30-fold higher than the therapeutic free concentration of clozapine in plasma water Ackenheil, M. et al. Arzneim-Forsch 26: 1156-1158 (1976); Sandoz Canada, Inc., Clozaril: Summary of preclinical and clinical data (1990)!. This suggests the existence of dopamine receptors more sensitive to the antipsychotic clozapine.

Van Tol et al., (1991) Nature, 350:610-614 report that they have cloned and sequenced a D2-like human dopamine receptor which they termed D4. They report that the dopamine D4 receptor gene has high homology to the human dopamine D2 and D3 receptor genes. They further report that the pharmacological profile of this receptor resembles that of the D2 and D3 receptors but that it has an affinity for clozapine which is tenfold higher. Van Tol et al. suggest that this characteristic may make the D4 receptor useful in discovering new types of drugs for schizophrenia that, like clozapine, do not induce tardive dyskinesia and other motor side effects.

Unfortunately, the anticipated low abundance of D4 receptors in the brain based on mRNA studies has hindered studies of the native D4 receptors. In addition, the genomic clone isolated and used by Van Tol et al. for expression contains introns. In a personal communication to the present inventors, H. H. M. Van Tol acknowledged that their expression construct could not be stably expressed. Therefore, a new approach is needed to facilitate the development of cell lines capable of the stable expression of a D4 dopamine receptor gene to further pharmacological and functional analysis of the D4 receptor.

INFORMATION DISCLOSURE

Van Tol, H. H. M., Bunzow, J. R., Guan, H.-C., Sunahara, R. K., Seeman, P., Niznik, H. B., and Civelli, O. (1991) Nature 350: 610-614.

Mills, A., Allet, B., Bernard, A., Chabert, C., Brandt, E., Cavegn, C., Chollet, A. and E. Kawashima (1993) FEBS Lett. 320: 130-134.

Horton, R. M., Hunt, H. D., Ho, S. N., Pullen, J. K., and Pease, L. R. (1989) Gene 77: 6168.

Sawadago, M., and VanDyke, M. W. (1991) Nucleic Acids Res. 19: 674.

Sambrook, J., Fritsch, E. F., and Maniatis, T. (1989) Molecular Cloning.

Yates, J. L., Warren, N., and Sugden, B. (1985) Nature 313: 812-815.

Van Tol, H. H. M., Wu, C. M., Guan, H.-C., Ohhara, K., Bunzow, J. R., Civelli, O., Kennedy, J., Seeman, P., Niznik, H. B., and Jovanovic, V. (1992) Nature 358: 149-152.

SUMMARY OF THE INVENTION

The present invention is directed toward the synthesis of a dopamine receptor corresponding to the nucleotide sequence SEQ ID NO: 1! shown in the top line of FIG. 2A, B, C, D, E, F. This nucleotide sequence encodes a synthetic gene for D4 dopamine receptor which has an identical amino acid sequence SEQ ID NO: 3! to the human D4 dopamine receptor. Therefore, the synthesized D4 dopamine receptor has drug dissociation properties similar to those properties characteristic of the human D4 dopamine receptor. For example, the synthetic gene expresses a D4 dopamine receptor which has an affinity for the drug clozapine similar to that displayed by human D4 dopamine receptor.

Synthesizing the human D4 dopamine receptor gene was surprisingly difficult. Attempts to use the polymerase chain reaction to obtain a full length clone for the D4 receptor were unsuccessful, possibly due to the naturally high G+C content of the human D4 dopamine receptor gene SEQ ID NO: 2!. This problem was surprisingly and unexpectedly overcome by using a chemo-enzymatic approach to construct the synthetic D4 dopamine receptor. While maintaining the published amino acid sequence SEQ ID NO: 3! for human D4 dopamine receptor, the human D4 dopamine receptor nucleic acid sequence SEQ ID NO: 2! was modified to reduce its G+C content to 50% from a natural level of 73%. This reduction allows the use of the polymerase chain reaction (PCR) in the synthesis of the synthetic D4 dopamine receptor. Using gene splicing by overlap extension and PCR amplification of long oligonucleotides (>200 mers), 3 synthetic fragments of about 400 base pairs (bp) each were amplified, from which the total gene was assembled. Stable expression of the synthetic gene was achieved in eukaryotic cells such as CHO-K1 cells, by use of an inducible system, and in HEK293 cells.

Transforming a eukaryotic cell culture with an expression vector containing the synthetic D4 dopamine receptor nucleic acid sequence provides for the expression of sufficient amounts of synthetic D4 dopamine receptor protein to allow for the convenient detection of dopamine binding inhibitors and also for the screening of compounds for anti-psychotic activity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E. Comparison of the modified D4 sequence to the original sequence.

The modified sequence (SynD4) SEQ ID NO: 1! is aligned with the original human sequence (HumD4) SEQ ID NO: 2!. The vertical lines indicate matching nucleotides, the bottom line shows the nucleotides that were changed to reduce the G+C content of the gene. Numbers at the ends of the lines correspond to nucleotide position.

FIGS. 2A, B, C, D, E, F. Nucleotide SEQ ID NO: 1! and amino acid SEQ ID NO: 3! sequences of the modified D4 gene.

The unique restriction sites are shown above the nucleotide sequence SEQ ID NO: 1!. Numbers at the ends of the lines indicate nucleotide position.

FIGS. 3A, B, C, D, E, F. Sequences of the templates and primers for the three sections of the modified D4 gene.

The sequence above the number line is the sense strand, and the antisense sequence is below the number line. Numbers at the ends of the lines correspond to the numbering in FIGS. 2A, B, C, D, E, F. Arrows at the 3' ends of the sequences indicate direction of nucleotide addition by Taq polymerase. All of the primers SEQ ID NOS 4-! have a HindIII site at their 5' ends for subcloning purposes.

FIG. 4. Scatchard analysis of (³ H)-spiperone binding in HEK 293 D4-24 cells.

Shown is a Scatchard transformation of the saturation analysis of the binding of ³ H-spiperone binding. Each point is the mean of duplicates with non-specific binding subtracted.

FIG. 5. Stable expression of D4 receptors in HEK 293 D4-24 cells.

³ H-spiperone binding at 540 pM was determined in membranes prepared from HEK 293 D4-24 cells harvested at the indicated passage.

FIG. 6. Inhibition of cAMP accumulation by dopamine in CHO cells expressing D4 receptors. Antagonism by clozapine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The term "synthetic or modified D4 dopamine receptor" as used herein refers to proteins homologous to the protein coded for by the nucleotide sequence depicted in the top line of FIGS. 1A to 1E. The synthetic D4 dopamine receptor has the same amino acid composition as the human D4 receptor. SEQ ID NO: 3!

Nucleotide bases are abbreviated herein as follows:

    ______________________________________                                         A-Adenine            G-Guanine                                                 C-Cytosine           T-Thymine                                                 ______________________________________                                    

Amino acid residues are abbreviated herein to either three letters or a single letter as follows:

    ______________________________________                                         Ala; A-Alanine     Leu; L-Leucine                                              Arg; R-Arginine    Lys; L-Lysine                                               Asn; N-Asparagine  Met; M-Methionine                                           Asp; D-Aspartic acid                                                                              Phe; F-Phenylalanine                                        Cys; C-Cysteine    Pro; P-Proline                                              Gln; Q-Glutamine   Ser; S-Serine                                               Glu; E-Glutamic Acid                                                                              Thr; T-Threonine                                            Gly; G-Glycine     Trp; W-Tryptophan                                           His; H-Histidine   Tyr; Y-Tyrosine                                             Ile; I-Isoleucine  Val; V-Valine                                               ______________________________________                                    

The production of proteins such as the D4 dopamine receptor from cloned genes by genetic engineering is well known. The synthetic D4 dopamine receptor may be synthesized in host cells transformed with vectors containing DNA encoding the synthetic D4 dopamine receptor. A vector is a replicable DNA construct. Vectors are used herein either to amplify DNA encoding the synthetic D4 dopamine receptor and/or to express DNA which encodes the synthetic D4 dopamine receptor SEQ ID NO: 1!. An expression vector is a replicable DNA construct in which a DNA sequence encoding the synthetic D4 dopamine receptor SEQ ID NO: 1! is operably linked to suitable control sequences capable of effecting the expression of the synthetic D4 dopamine receptor in a suitable host. The need for such control sequences will vary depending upon the host selected and the transformation method chosen. Generally, control sequences include a transcriptional promoter, an optional operator sequence to control transcription, a sequence encoding suitable mRNA fibosomal binding sites, and sequences which control the termination of transcription and translation. Amplification vectors do not require expression control domains. All that is needed is the ability to replicate in a host, usually conferred by an origin of replication, and a selection gene to facilitate recognition of transformants.

Vectors useful for practicing the present invention include plasmids, viruses (including phage), retroviruses, and integratable DNA fragments (i.e., fragments integratable into the host genome by homologous recombination). The vector replicates and functions independently of the host genome, or may, in some instances, integrate into the genome itself. Suitable vectors will contain replicon and control sequences which are derived from species compatible with the intended expression host. Transformed host cells are cells which have been transformed or transfected with the synthetic D4 dopamine receptor vectors constructed using recombinant techniques. Transformed host cells ordinarily express the synthetic D4 receptor, but host cells transformed for purposes of cloning or amplifying the synthetic D4 receptor DNA need not express the synthetic D4 receptor. When expressed, the synthetic D4 receptor will typically be located in the host cell membrane.

DNA regions are operably linked when they are functionally related to each other. For example: a promoter is operably linked to a coding sequence if it controls the transcription of the sequence; a ribosome binding site is operably linked to a coding sequence if it is positioned so as to permit translation. Generally, operably linked means contiguous and, in the case of leader sequences, contiguous and in the same translational reading frame.

Cultures of cells derived from multicellular organisms are a desirable host for recombinant synthetic D4 dopamine receptor synthesis. In principle, any higher enkaryotic cell culture is workable, whether from vertebrate or invertebrate culture. However, mammalian cells are preferred, as illustrated in the Examples. Propagation of such cells in cell culture has become a routine procedure. See Tissue Culture, Academic Press, Kruse and Patterson, editors (1973). Examples of useful host cell lines are Chinese hamster ovary (CHO) cell lines and HEK293 cells. Expression vectors for such cells ordinarily include (if necessary) an origin of replication, a promoter located upstream from the gene to be expressed, along with a ribosome binding site and a polyadenylation site for transcriptional termination.

The transcriptional and translational control sequences in expression vectors to be used in transforming vertebrate cells are often provided by mammalian and vital sources. For example, commonly used promoters can be derived from polyoma, adenovirus 2, and simian virus 40 (SV 40). Further, the human genomic D4 receptor promoter, control and/or signal sequences may also be used provided such control sequences are compatible with the host cell chosen. In the current invention, both inducible and constitutive expression were tested.

An origin of replication may be provided either by construction of the vector to include an exogenous origin, such as may be derived from SV40 or other vital source (e.g., polyoma, adenovirus, VSV, or MPV), or may be provided by the host cell chromosomal replication mechanism. If the vector is integrated into the host cell chromosome, the latter may be sufficient.

D4 dopamine receptors made from synthetic genes in accordance with the present invention may be used for screening compounds for D4 dopamine receptor activity, or for determining the amount of a dopaminergic drug in a solution (e.g., blood plasma or serum). For example, host cells may be transformed with a vector of the present invention, D4 dopamine receptors expressed in that host, the cells lysed, and the membranes from those cells used to screen compounds for D4-dopamine receptor activity. Competitive binding assays in which such procedures may be carried out are well known. Intact cells may also be used to measure D4 receptor activation. By selection of host cells which do not ordinarily express a dopamine receptor, preparations of membranes containing D4 receptors made from synthetic genes can be obtained. Membranes obtained from such cells can be used in binding studies wherein the drag dissociation activity is monitored. Further, D4 dopamine receptor agonist and antagonists can be identified by procedures well known in the art.

Cloned genes and vectors of the present invention are useful to transform cells which do not ordinarily express the D4-dopamine receptor to thereafter express this receptor. Such cells are useful as intermediates for making cell membrane preparations useful for receptor binding assays, which are in turn useful for drug screening.

Initial attempts by the present inventors to use the polymerase chain reaction (PCR) to amplify regions of the D4 gene SEQ ID NO: 2! from human genomic DNA and from first-strand cDNA prepared from SK-N-MC cells were unsuccessful probably because the D4 receptor gene has an inordinately high G+C content (73%). The high G+C content may make this gene prone to formation of very stable secondary structures or difficult to completely denature, either of which may result in inefficient PCR amplification. Attempts to overcome these obstacles using variations of the PCR, such as co-solvents (10% DMSO or 10% glycerol), base analogues (7-deaza-dGTP), and high-temperature annealing were also unsuccessful, even to obtain short lengths of DNA product (about 200 bp). To solve this problem, the present inventors designed and synthesized by chemo-enzymatic methods a modified gene coding for a dopamine D4 receptor SEQ ID NO: 1!. The gene was synthesized using the techniques of gene splicing by overlap extension described in Horton et al. (1989) Gene 77:61-68 (the disclosure of which is herein incorporated by reference) with PCR amplification of long nucleotides. The synthetic D4 receptor gene SEQ ID NO: 1! has a lowered G-C content and no introns, can be used to stably express synthetic D4 dopamine receptors in cultures of transformed eukaryotic cells, and retains the drag dissociation properties characteristic of human dopamine receptor D4. SEQ ID NO: 3! The stable expression of these synthetic receptors provides for a ready supply of transformed eukaryotic cells producing the synthetic dopamine receptor for the characterization of antipsychotic drugs.

The Examples which follow are illustrative of specific embodiments of the invention, and various uses thereof. They are set forth for explanatory purposes only, and are not to be taken as limiting the invention.

Example 1 Gene Design

To modify the gene for the D4 receptor SEQ ID NO: 2!, the SILENT program (GCG DNA Sequence Analysis Software, Madison, Wis.) was used to find potential unique restriction sites in the original gene. The sequence of the original gene was then changed to incorporate additional unique sites approximately every 50 base pairs (bp). The amino acid sequence of the receptor SEQ ID NO: 3! was unaltered. The G+C content of the gene was then decreased by exchanging nearly every G or C for an A or T while maintaining the original amino acid sequence SEQ ID NO: 3! and the unique restriction sites. The sequence was then searched for consensus splice donor (CAGGRRRT) and acceptor ((Y)₁₃ AGG) sites and polyadenylation (AWTAAAA) signals and none were found. A consensus translation initiation signal (CCACC) was added to the 5' end immediately upstream of the initiation codon ATG.

The modified nucleotide sequence (SynD4) SEQ ID NO: 1! is compared with the original human D4 sequence (HumD4) SEQ ID NO: 2! in FIGS. 1A and B. The line labeled Diff. shows the bases that were changed to lower the G+C content of the gene. The final G+C content of the gene was 50%, compared to 73% for the coding region of the original sequence. Decreasing the G+C content allowed the use of the polymerase chain reaction (PCR) in the synthesis of the D4 gene. The nucleotide SEQ ID NO: 1! and amino acid SEQ ID NO: 3! sequence of the modified gene is shown in FIGS. 2A, B, C, D, E, F with the positions of the unique restriction sites. The amino acid sequence is identical to that of the published D4 receptor (shown at the third line of FIGS. 1A to 1E). The unique sites can be used for mutagenesis and structure-function analysis of the receptor. The HindIII sites at the 5' and 3' ends of the complete gene used for subcloning are not shown.

Example 2 Oligonucleotide Synthesis

For synthesis, the synthetic D4 gene (1169 bp) SEQ ID NO: 1! was divided into 3 sections of ˜400 bp each. Two long oligonucleotide templates (oligos) greater than 200 bp each were synthesized on a 0.2 μmole scale on a 380B DNA Synthesizer (Applied Biosystems, Inc.) using β-cyanoethyl phosphoramidites. Release of the trityl protecting group during synthesis was monitored every 25 cycles to determine coupling efficiencies. The oligos were deprotected in concentrated NH₄ OH for 24 hours at 65° C. and purified by n-butanol extraction and ethanol precipitation as described in Sawadago, M., and Van Dyke, M. W. (1991) Nucleic Acids Res. 19: 674. The oligos were synthesized to include the 5' sense and 3' antisense sequence of half of each section. These templates were complementary at their 3' ends (˜20 bp overlap) so that when annealed and extended with Taq polymerase, a double stranded DNA of ˜400 bp was created. Two primers complementary to the 3' ends of the newly synthesized strands were then used to amplify the full length double stranded (.sup.˜ 400 bp) DNA. The sequences of the templates and primers used to synthesize each of the sections are shown in FIGS. 3A, B, C, D, E, F.

Example 3 Polymerase Chain Reactions

GeneAmp PCR kits from Perkin Elmer Cetus were used. Temperature cycling was done in a Perkin Elmer Cetus DNA Thermal Cycler. For each section of the gene to be amplified, a 100 μl standard reaction was prepared with 1 μg of each oligo (2 templates and 2 primers). The template sequences were amplified by annealing at 60° C. for 1 minute, extending at 72° C. for 1 minute, and denaturing at 94° C. for 1 minute (3 cycles) followed by 22 cycles as above except annealing was done at 65° C. Final PCR products were digested with HindIII, gel purified (Gene Clean), and subcloned into Bluescript SK(+) for sequencing. Standard DNA manipulations were performed as described in Sambrook, J., Fritsch, E. F., and Maniatis, T. (1989) Molecular Cloning.

Example 4 Nucleotide Sequencing and Analysis of PCR Generated Products

Double stranded plasmid DNA was sequenced by the enzymatic method using T7 polymerase from U.S. Biochemicals (Sequenase). Oligonucleotide primers were located in Bluescript (flanking the insert) or were the same as used in the PCR amplification reactions. Sequence reactions were electrophoresed on one meter thermosrated gels prepared with acrylamide or Hydrolink. From 12 clones with inserts of the 5' section, four had the correct sequence. All seven clones of the middle section of the gene selected for sequencing had errors, but one clone contained only a single C to T transversion that is silent (no amino acid change). From eight clones of the 3' section of the gene, one was correct.

Example 5 Assembly of Synthetic D4 Gene

Insert DNA was isolated from clones containing correct sequence for each of the three sections of the gene (the 5' insert was cleaved with HindIII and ClaI, the middle insert was cleaved with ClaI and PflMI, and the 3' insert was cleaved with PflMI and HindIII). The DNA inserts (1 μg of each insert) were gel purified and ligated together in a 10 μl reaction (16 hrs at 16° C.). Ligation was stopped by heating the reaction at 75° C. for 10 minutes. After cooling on ice, the ligated products were digested with 20 units of HindIII at 37° C. for 1 hour to give a full length gene of 1169 bp SEQ ID NO: 1!. After agarose gel electrophoresis, the DNA was isolated and subcloned into Bluescript SK(+).

Example 6 D4 Receptor Expression Constructs

The synthetic D4 gene SEQ ID NO: 1! was isolated from Bluescript SK(+) by cleavage with HindIII and purified by agarose gel electrophoresis. Initial attempts to stably express the synthetic D4 gene SEQ ID NO: 1! in CHO pro⁻ 5 cells were unsuccessful, although these cells express D2 and D3 receptors quite well. We hypothesized that if synthetic D4 receptor expression were affecting cell growth or viability, an inducible expression system may allow for the isolation of stably transfected cell lines. The CHO LacI line and vector system was used for this purpose. The synthetic D4 gene SEQ ID NO: 1! was subcloned into pGG668, an expression vector that contains a lac repressor binding site immediately downstream from the TATA box of the CMV promoter, both of which are 5' to unique HindIII and BamHI cloning sites. Orientation of the clone was determined by BamHI digestion. This vector also encodes a gene conferring resistance to hygromycin B. When this vector is used in CHO-K1 cells that have been engineered to constitutively express a bacterial lac repressor (LacI cells), gene expression can be induced by adding 100 μM isopropyl thiogalatoside (IPTG) to the growth medium. The synthetic D4 gene SEQ ID NO: 1! was also subcloned into the HindIII site of the EBV-based vector pCEP4 (Invitrogen) for expression in HEK293 cells. This vector has a CMV promoter 5' to the cloning site, and encodes a gene conferring resistance to hygromycin B. This vector has been shown to replicate episomally at high copy numbers in this cell line (Yates, J. L., Warren, N., and Sugden, B. (1985) Nature 313:812-815). Orientation of the clone was determined by BamHI digestion.

Example 7 Isolation of Stable Transfectants

CHO LacI cells were gown in αMEM, 2 mM L-glutamine, 100 U/ml penicillin G, 100 μg/ml streptomycin, 10% fetal bovine serum, and 500 μg/ml G418 (to maintain expression of the lac repressor). HEK293 cells were gown in DMEM high glucose, 4 mM L-glutamine, 100 U/ml penicillin G, 100 μg/ml streptomycin, and 10% fetal bovine serum. Both lines were gown in an atmosphere of 5% CO₂ at 37° C. CHO LacI cells and HEK293 cells were transfected with the appropriate expression construct with the liposome reagent DOTAP (BM), following the supplier's protocol. LacI cells were selected with 500 μg/ml G418 and 500 u/ml hygromycin B (Calbiochem). HEK293 cells were selected with 100 u/ml hygromycin B. After several weeks of selection, resistant colonies were isolated and gown for binding analysis.

Example 8 Preparation of Membranes

LacI synthetic D4 transfected cells were grown in the presence of 100 μM IPTG for 48 hours prior to harvest. Cells gown in 100 mm dishes were rinsed once with 5 mls of ice cold Ca²⁺ /Mg²⁺ free PBS and scraped into 5 mls of the same buffer. Cells were pelleted (500 x g, 5 minutes), and resuspended in 25 mM Tris, 5 mM EDTA, 5 mM EGTA, pH 7.5 (TEE), and frozen in liquid nitrogen. After thawing, the cells were homogenized and centrifuged at 1,000 x to remove nuclei and unbroken cells. The supernatant was centrifuged at 20,000 x g, the membrane pellet was washed once with TEE, resuspended in binding buffer, and frozen in liquid nitrogen. Membrane aliquots were stored at -70° C.

Example 9 Radioligand Binding Assay

Binding assays were carried out in a final volume of 1 ml of 20 mM HEPES, pH 7.5, 150 mM NaCl, 10 mM MgCl₂, 1 mM EDTA, including (³ H)spiperone (Amersham, 119 Ci/mmol) and vehicle or 20 μM (+)butaclamol, to determine nonspecific binding. Incubations were for 1 hour at 30° C. Assays were stopped by filtration through Whatman GF/B filters followed by 2×10 ml washes with ice cold 50 mM Tris, pH 7.5. The filters were counted by liquid scintillation spectroscopy.

Twenty-four hygromycin resistant LacI cell lines which had been induced to express synthetic D4 receptors with IPTG were assayed by single-point binding analysis, with expression ranging from ˜20 to 236 fmol/mg protein of (³ H)-spiperone binding sites. The highest expressing line, D4-19, was selected for saturation binding analysis. Scatchard transformation of the data gave a K_(d) of 158 pM and a B_(max) of 260 fmol/mg protein. To determine if sustained synthetic D4 receptor expression was detrimental to cell growth or viability we continuously induced synthetic D4 receptor expression in the D4-19 line for 27 days with fresh media containing 100 μM IPTG added every 2-4 days. Continuous induction had no effect on receptor density, which was maintained at values between 227 fmol/mg protein to 330 fmol/mg protein over the course, assayed every 2-4 days. This indicated that expression of the synthetic D4 receptor was not limited to inducible expression systems.

Thirty-eight hygromycin resistant HEK293 lines were assayed in the same manner as the LacI lines and expression ranged from ˜20 to 720 fmol/mg protein (³ H)-spiperone binding sites. The highest expressing line, D4-24, was selected for saturation binding analysis with ³ H!spiperone. Scatchard analysis of the data gave a K_(d) of 100 pM and a B_(max) of 736 fmol/mg protein (FIG. 4). Synthetic D4 receptor expression in the D4-24 cell line remains constant for up to at least 12 passages of the cells, over a period of 10 weeks (FIG. 5).

Example 10 Functional Assay for D4 Inhibitors - cAMP Assays

The synthetic D4 gene SEQ ID NO: 1! can also be used in other assays to screen compounds as inhibitors of D4 receptor activation. In these assays, a compound is added to a transformed eukaryotic cell and an assay is performed to detect a D4 receptor mediated signaling event such cAMP inhibition, a change in the extracellular acidification rate, or a change in the lipid metabolism of the cell, for example, arachidonic acid secretion. If there is no effect on the D4 receptor mediated signaling event, then a test is performed to test the compound for its ability to inhibit the D4 receptor signaling event. One such assay is the inhibition of cAMP accumulation by dopamine in D4-19 Chinese Hamster Ovary cells expressing D4 receptors. In this assay, clozapine is used as the antagonist.

AMP accumulation was measured in intact D4-19 cells plated at a density of 15,000 cells/well in a 24 well plate 48 hours prior to the experiment. The cells were incubated in serum free medium 1 hour prior to the experiment. Fresh medium (0.5 ml) containing 100 μM forskolin, 100 μM IBMX and varying concentrations of drugs were added to each well and cAMP was allowed to accumulate for 15 minutes at 30° C. The reactions were terminated by the removal of the medium and the addition of 100 μl cold 7.5% (TCA) trichloroacetic acid. The samples were diluted by the addition of 1.0 ml 50 mM sodium acetate, pH 6.2 and aliquots were assayed by RIA using Biomedical Technologies Incorporated cAMP RIA kit.

cAMP accumulation was measured in forskolin stimulated cells as described in methods. Dopamine (10 μM) was added to 3 wells (DA). Dopamine (10 μM) and clozapine (10 μM) were both added to 3 wells (DA+CLOZ) and clozapine alone (10 μM) was added to 3 wells (CLOZ). The bars are the averages of the three wells expressed as the percentage of the forskolin response.

In summary, we have synthesized a modified gene that codes for the human D4 dopamine receptor SEQ ID NO: 3!. The synthetic gene SEQ ID NO: 1! has been stably expressed in CHO cells using an inducible expression system, and in HEK293 cells constitutively. The stable expression of this modified gene will facilitate pharmacological and functional studies of the D4 receptor.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 9                                                   (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 1161 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        GGTAACAGATCTACTGCAGACGCAGACGGACTACTTGCTGGAAGAGGTCCAGCTGCAGGA60                 GCTTCTGCTGGAGCATCCGCTGGACTTGCTGGTCAAGGAGCTGCAGCTTTAGTTGGTGGA120                GTGTTGTTAATTGGTGCAGTATTAGCTGGAAACAGTCTAGTTTGTGTTAGTGTAGCAACT180                GAAAGAGCTCTACAAACACCTACTAACTCATTCATAGTAAGTTTAGCAGCTGCAGATTTA240                TTGCTAGCTTTACTTGTACTACCATTATTTGTTTACTCAGAAGTCCAGGGTGGAGCTTGG300                CTCTTAAGTCCAAGATTGTGTGACGCACTCATGGCAATGGACGTCATGTTGTGTACTGCT360                TCAATTTTCAACTTGTGTGCAATCAGTGTAGATCGATTCGTAGCAGTTGCTGTACCATTA420                AGATACAACAGACAAGGAGGCAGTCGACGCCAGCTGCTTCTCATTGGAGCTACATGGTTA480                CTATCAGCGGCCGTAGCTGCACCTGTACTGTGTGGATTGAATGATGTTAGAGGAAGGGAT540                CCAGCTGTATGTAGATTGGAAGATAGAGATTATGTAGTTTACTCGAGTGTATGTAGTTTC600                TTTCTACCATGTCCATTAATGTTGCTACTTTACTGGGCAACATTCAGAGGACTGCAGAGA660                TGGGAAGTTGCACGTCGTGCAAAGTTACATGGAAGAGCACCAAGAAGACCTAGTGGACCT720                GGACCACCTAGTCCAACACCTCCAGCTCCAAGATTACCTCAAGATCCTTGTGGGCCCGAC780                TGTGCTCCTCCAGCACCTGGTTTACCTCCAGATCCTTGTGGTTCTAACTGTGCTCCTCCA840                GATGCGGTCCGAGCTGCAGCTCTACCACCTCAGACTCCACCTCAGACGCGTAGGAGAAGG900                AGAGCTAAGATCACAGGAAGGGAGAGGAAAGCTATGAGGGTCCTACCTGTGGTCGTTGGT960                GCATTCTTGTTATGCTGGACACCATTCTTTGTTGTGCACATCACACAAGCACTGTGTCCT1020               GCTTGCTCAGTGCCACCTAGACTTGTCAGTGCGGTCACCTGGTTGGGTTACGTAAACTCA1080               GCACTCAATCCTGTCATCTACACTGTATTCAATGCTGAATTCAGAAATGTCTTCAGAAAG1140               GCTCTAAGAGCTTGCTGCTGA1161                                                      (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 1161 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        GGGAACCGCAGCACCGCGGACGCGGACGGGCTGCTGGCTGGGCGCGGGCCGGCCGCGGGG60                 GCATCTGCGGGGGCATCTGCGGGGCTGGCTGGGCAGGGCGCGGCGGCGCTGGTGGGGGGC120                GTGCTGCTCATCGGCGCGGTGCTCGCGGGGAACTCGCTCGTGTGCGTGAGCGTGGCCACC180                GAGCGCGCCCTGCAGACGCCCACCAACTCCTTCATCGTGAGCCTGGCGGCCGCCGACCTC240                CTCCTCGCTCTCCTGGTGCTGCCGCTCTTCGTCTACTCCGAGGTCCAGGGTGGCGCGTGG300                CTGCTGAGCCCCCGCCTGTGCGACGCCCTCATGGCCATGGACGTCATGCTGTGCACCGCC360                TCCATCTTCAACCTGTGCGCCATCAGCGTGGACAGGTTCGTGGCCGTGGCCGTGCCGCTG420                CGCTACAACCGGCAGGGTGGGAGCCGCCGGCAGCTGCTGCTCATCGGCGCCACGTGGCTG480                CTGTCCGCGGCGGTGGCGGCGCCCGTACTGTGCGGCCTCAACGACGTGCGCGGCCGCGAC540                CCCGCCGTGTGCCGCCTGGAGGACCGCGACTACGTGGTCTACTCGTCCGTGTGCTCCTTC600                TTCCTACCCTGCCCGCTCATGCTGCTGCTCTACTGGGCCACGTTCCGCGGCCTGCAGCGC660                TGGGAGGTGGCACGTCGCGCCAAGCTGCACGGCCGCGCGCCCCGCCGACCCAGCGGCCCT720                GGCCCGCCTTCCCCCACGCCACCCGCGCCCCGCCTCCCCCAGGACCCCTGCGGCCCCGAC780                TGTGCGCCCCCCGCGCCCGGCCTCCCCCCGGACCCCTGCGGCTCCAACTGTGCTCCCCCC840                GACGCCGTCAGAGCCGCCGCGCTCCCACCCCAGACTCCACCGCAGACCCGCAGGAGGCGG900                CGTGCCAAGATCACCGGCCGGGAGCGCAAGGCCATGAGGGTCCTGCCGGTGGTGGTCGGG960                GCCTTCCTGCTGTGCTGGACGCCCTTCTTCGTGGTGCACATCACGCAGGCGCTGTGTCCT1020               GCCTGCTCCGTGCCCCCGCGGCTGGTCAGCGCCGTCACCTGGCTGGGCTACGTCAACAGC1080               GCCCTCAACCCCGTCATCTACACTGTCTTCAACGCCGAGTTCCGCAACGTCTTCCGCAAG1140               GCCCTGCGTGCCTGCTGCTGA1161                                                      (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 387 amino acids                                                    (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        MetGlyAsnArgSerThrAlaAspAlaAspGlyLeuLeuAlaGlyArg                               151015                                                                         GlyProAlaAlaGlyAlaSerAlaGlyAlaSerAlaGlyLeuAlaGly                               202530                                                                         GlnGlyAlaAlaAlaLeuValGlyGlyValLeuLeuIleGlyAlaVal                               354045                                                                         LeuAlaGlyAsnSerLeuValCysValSerValAlaThrGluArgAla                               505560                                                                         LeuGlnThrProThrAsnSerPheIleValSerLeuAlaAlaAlaAsp                               65707580                                                                       LeuLeuLeuAlaLeuLeuValLeuProLeuPheValTyrSerGluVal                               859095                                                                         GlnGlyGlyAlaTrpLeuLeuSerProArgLeuCysAspAlaLeuMet                               100105110                                                                      AlaMetAspValMetLeuCysThrAlaSerIlePheAsnLeuCysAla                               115120125                                                                      IleSerValAspArgPheValAlaValAlaValProLeuArgTyrAsn                               130135140                                                                      ArgGlnGlyGlySerArgArgGlnLeuLeuLeuIleGlyAlaThrTrp                               145150155160                                                                   LeuLeuSerAlaAlaValAlaAlaProValLeuCysGlyLeuAsnAsp                               165170175                                                                      ValArgGlyArgAspProAlaValCysArgLeuGluAspArgAspTyr                               180185190                                                                      ValValTyrSerSerValCysSerPhePheLeuProCysProLeuMet                               195200205                                                                      LeuLeuLeuTyrTrpAlaThrPheArgGlyLeuGlnArgTrpGluVal                               210215220                                                                      AlaArgArgAlaLysLeuHisGlyArgAlaProArgArgProSerGly                               225230235240                                                                   ProGlyProProSerProThrProProAlaProArgLeuProGlnAsp                               245250255                                                                      ProCysGlyProAspCysAlaProProAlaProGlyLeuProProAsp                               260265270                                                                      ProCysGlySerAsnCysAlaProProAspAlaValArgAlaAlaAla                               275280285                                                                      LeuProProGlnThrProProGlnThrArgArgArgArgArgAlaLys                               290295300                                                                      IleThrGlyArgGluArgLysAlaMetArgValLeuProValValVal                               305310315320                                                                   GlyAlaPheLeuLeuCysTrpThrProPhePheValValHisIleThr                               325330335                                                                      GlnAlaLeuCysProAlaCysSerValProProArgLeuValSerAla                               340345350                                                                      ValThrTrpLeuGlyTyrValAsnSerAlaLeuAsnProValIleTyr                               355360365                                                                      ThrValPheAsnAlaGluPheArgAsnValPheArgLysAlaLeuArg                               370375380                                                                      AlaCysCys                                                                      385                                                                            (2) INFORMATION FOR SEQ ID NO:4:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 33 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                        CACTATAAGCTTCCACCATGGGTAACAGATCTA33                                            (2) INFORMATION FOR SEQ ID NO:5:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 33 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                        AGTCACATCTAGCTAAGCATCTTCGAATATCAC33                                            (2) INFORMATION FOR SEQ ID NO:6:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 33 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                        CACTATAAGCTTATCGATTCGTAGCAGTTGCTG33                                            (2) INFORMATION FOR SEQ ID NO:7:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 31 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                        CACGAGGAGGTCGTGGACCTTCGAATATCAC31                                              (2) INFORMATION FOR SEQ ID NO:8:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 32 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                        CACTATAAGCTTCCAGCACCTGGTTTACCTCC32                                             (2) INFORMATION FOR SEQ ID NO:9:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 35 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                        TTCCGAGATTCTCGAACGACGACTTCGAATATCAC35                                          __________________________________________________________________________ 

What we claim is:
 1. A DNA encoding a dopamine receptor, wherein said DNA is characterized as comprising the nucleotide sequence of SEQ ID NO:1.
 2. A DNA according to claim 1 wherein the dopamine receptor encoded thereby is a D4 dopamine receptor.
 3. A cloning vector comprising a DNA encoding a dopamine receptor, wherein said DNA is characterized as comprising the nucleotide sequence of SEQ ID NO:1.
 4. A recombinant expression vector comprising the DNA of claim 1, wherein the vector expresses the synthetic dopamine receptor D4 in a transformed eukaryotic cell culture.
 5. A eukaryotic cell culture transformed with the expression vector of claim 4, wherein the transformed eukaryotic cell culture expresses the synthetic dopamine receptor D4. 